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Why do some geomagnetic storms repeat on a 27-day cycle?

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Diagram of Earth's atmosphere layers-troposphere, stratosphere, mesosphere, and thermosphere-showing how temperature and air pressure change with increasing altitude, with key features like the ozone layer and pressure percentages at various heights. — Ionosphere and magnetosphere The layers of Earth's atmosphere, with a yellow line showing the air temperature at various heights.

Geomagnetic storms that repeat on a 27-day cycle are known as recurrent storms. These storms are caused by coronal holes on the Sun, which are regions in the solar atmosphere where gas density is lower and charged particles escape more easily.1

These coronal holes produce high-speed streams of solar wind that interact with the slower-speed solar wind emitted from other regions of the Sun. This interaction creates high-pressure areas known as corotating interaction regions. As the Sun rotates, these coronal holes and their associated high-speed streams persist for several months, causing the resulting disturbances in the solar wind to sweep past Earth repeatedly with the Sun's 27-day equatorial rotation period. When these streams tilt the interplanetary magnetic field into an orientation antiparallel to Earth's magnetic field, they trigger recurrent geomagnetic storms in the magnetosphere.12